Impedance matching systems have been more and more widely and gradually used several decades. Generally, an impedance matching system includes a host and display connected with the host via a transmission line. The display can be a traditional CRT (cathode ray tube) display, a prevailing LCD (liquid crystal display), or other type display devices. The transmission line can be a metal wire or any other lines that can transmit signals between the host and the display.
Referring to FIG. 3, a typical impedance matching system includes a host 11, a transmission line 13, a display device 15, and a resistor 17. The host 11 includes a processor 111, a display controller 113 and a display signal output terminal 115. The display device 15 includes a display signal input terminal 151, a processing unit 153, and a screen 155. The display signal output terminal 115 is connected to the display signal input terminal 151 via the transmission line 13. The resistor 17 is connected between an end of the transmission line 13, adjacent to the display signal input terminal 151, and ground.
The processor 111 provides various data signals to the display controller 113. The display controller 113 generates a plurality of display signals according to the data signals, and sends the display signals to the display signal output terminal 115. The display signal input terminal 151 receives the display signals from the display signal output terminal 115 via the transmission line 13, and then sends the display signals to the processing unit 153 for a further processing procedure, which are finally applied to the screen 155 for image display.
The transmission line 13 is usually marked with a characteristic impedance which represents a reference impedance adapted to most situations. When a resistance of the resistor 17 is equal to the characteristic impedance, the impedance of the transmission line 13 is matched or closely matched. In this case, percentages of loss of the display signals caused by signal reflection at ends of the transmission line 13 can be reduced or eliminated.
However, an actual impedance of the transmission line 13 is variable and not equal to the characteristic impedance in most cases. When different display signals having different frequencies are applied to the transmission line 13, the actual impedance of the transmission line 13 is correspondingly different. Therefore, the resistance of the resistor 17 and the actual impedance of the transmission line 13 are not matched precisely. In order to match the resistance with the resistor 17 to the impedance of the transmission line 13, manual operation is adopted to determine the resistance of the resistor 17 with help of a test instrument (not shown). The test instrument is capable of testing whether the transmission of the display signal is in a best mode, and the resistance of the resistor 17 is determined if the transmission of the display signal is in a best mode. Therefore, the actual impedance of the transmission line 13 and the resistance of the resistor 17 are matched. Although by practicing the above-described impedance matching method, impedance matching can be achieved, the operation of determining the resistance of the resistor 17 is complicated and troublesome.
What is needed, therefore, is an impedance matching method which can overcome the above-described deficiencies. Which are also needed, are an impedance matching circuit and an impedance matching system employing such impedance matching circuit.